1. Field of the Invention
The present invention relates to partial SOI substrate having an SOI (silicon-on-insulator) structure element manufactured by the SIMOX (separation by implanted oxygen) technique and a bulk structure element mounted as mixed thereon and a method for the production of a partial SOI substrate.
2. The Prior Art
Heretofore, in the production of a partial SOI by the SIMOX, it has been customary to inject oxygen ions in a fixed direction to the opening part of a mask owing to the construction of an ordinary SIMOX grade ion implanter.
Specifically, this procedure has been directed to accomplishing the formation of a buried oxide film 5 by implanting oxygen ions into the surface of the substrate through an opening part 3 formed in a protective film 2 by lithography, subjecting the affected substrate to a prescribed washing treatment, and performing a heat treatment at an elevated temperature thereon (refer to Patent Document 1 and Patent Document 2).
In these Patent Documents 1 and 2, the idea of inclining the implantation by an angle of about 7 degrees relative to the normal of the surface of the substrate is introduced. This inclination is aimed at adjusting the distribution of implanted oxygen ions thereby ensuring formation of a perfect buried oxide film. This inclined ion implantation, however, results in exposing the edge of the buried oxide film through the surface (refer to Patent Document 2 and Patent Document 3). The reason offered to explain this phenomenon is that since the part embracing the opening part is not thoroughly shielded against ions by the protective film as illustrated in FIG. 4 and oxygen ions are inevitably implanted into the surface of the substrate. The buried oxide film is eventually exposed to the surface when the buried oxide film is formed by the heat treatment. Since this exposed part inevitably results in forming a dent or a cavity in the subsequent step of removing the oxide film by washing the surface with hydrofluoric acid, it constitutes a hindrance to the circuit design. This is due to necessitating the incorporation of a device-isolation structure without fail and poses a problem regarding the aspect of the process as by entailing the occurrence of excess slurry in the step of CMP.
Patent Document 2 and Patent Document 3 propose measures for the solution of these problems. These measures, however, are not easily applied to actual commercial production.
The perpendicular implantation proposed in Patent Document 2 interferes with the formation of BOX and degrades the quality of the BOX because the channeling gives rise to a tail in the distribution of implanted oxygen. The anisotropic etching performed at an inclination of 7-10 degrees from the normal of the surface of the substrate as proposed in Patent Document 3 permits no easy control. This is because even the ECR method adopts a principle of causing the potential difference between the plasma and the substrate to occur in a direction perpendicular to the surface of the substrate. The idea of utilizing the resonance point fails to secure the directional property because the inclination gives rise to a deviation. The process using a nitride film is at a disadvantage in increasing the number of stages of work, elongating the time of work, and adding to the cost of production.
The partial SOI proposed to date inevitably results in a step between the SOI region and the non-SOI region. No procedure has yet been proposed for the solution of this problem and no effort has been made with the object of exploring the clear existence of this problem. The tolerated step depends on the circuit pattern dimensioning of a target integrated circuit. Although it is said to be not more than 200 nm or thereabout, the magnitude of the allowable step tends to decrease.
Official gazette of JP-A-08-017694 [Patent Document 1]
Official gazette of JP-A-2001-308025 [Patent Document 2]
Official gazette of JP-A-2001-308172 [Patent Document 3]
Heretofore, in the production of a partial SOI substrate by the SIMOX technique, the ordinary method inevitably requires the exposure of BOX and several measures proposed for the solution of this problem invariably encounter difficulty in achieving commercial utilization.